In a bicycle, a chain runs between a front sprocket and a rear sprocket. The chain is gradually stretched with use and may be removed from the sprockets. In order to prevent this from happening, a mechanism for applying tension to the chain is known. In a Japanese so-called light roadster, tension can be applied to a chain by forming a rear pawl of a normal pawl type opening rearward in a rear end of a frame, and moving a hub shaft rearward.
On the other hand, a rear pawl of a reverse pawl type opening diagonally forward and downward is formed in a rear end of a frame used in a mountain bike or the like. To a chain mounted to the frame having such a rear pawl of the reverse pawl type, tension is generally applied by a rear derailleur that is an external shift device. However, an internal shift hub as an internal shift device using a planetary gear train has recently included a larger number of stages, and an internal shift hub having no chain tension applying function is sometimes mounted to a frame having a rear pawl of a reverse pawl type. For example, when an internal shift hub and a disk brake are to be used together, a frame for a mountain bike having a brake seat is often used, and in this case, the internal shift hub is mounted to the frame of the reverse pawl type. For a frame having a rear suspension, the rear suspension generally has the center of rotation in a position spaced apart from a hub shaft to which a front sprocket is mounted. Thus, rotation of the rear suspension may change a distance between the front sprocket and the rear sprocket to cause the chain to be removed from the sprockets. In such a case, a chain tension applying device is required for accommodating a stretch of the chain and the change in the distance between the sprockets.
A conventionally known chain tension applying device is placed around a motor sprocket mounted to a motor of an electric assist bicycle (for example, see Japanese Patent Laid-Open No. 2005-075069). The conventional chain tension applying device is pivotally mounted to a fixed arm secured by a screw to a housing of a drive unit accommodating the motor. The chain tension applying device includes a spring housing bracket pivotally mounted to the fixed arm, a coil spring having one end locked to the spring housing bracket, and a support bracket that is retractably mounted to the spring housing bracket, has a tension sprocket in a tip thereof, and is urged toward a retracted side by the coil spring.
Mounting such a conventional chain tension applying device to a frame around a rear sprocket or a front sprocket prevents a chain from being removed from the sprockets even if the chain is stretched or a distance between the sprockets changes.
The conventional chain tension applying device is mounted to the housing of a motor drive via the fixed arm around the motor sprocket. The conventional chain tension applying device having such a structure is mounted to the housing of the drive unit to which the motor is mounted via a fixed bracket, which causes no change in an axial distance between the chain tension applying device and the motor sprocket, that is, a distance in a direction perpendicular to a tension direction. Thus, placing the conventional chain tension applying device on the frame around the front sprocket or the rear sprocket may prevent the change in the axial distance between the frame and the sprocket from being accommodated. Thus, chain tension applying devices need to be produced having different sizes according to axial sizes of a rear hub to which the frame and the rear sprocket are mounted and a crankshaft to which the front sprocket is mounted.